Hα photometry of low-mass stars in 47 Tucanae: chromospheric activity and exotica

We have used archival Hubble Space Telescope observations obtained with the Advanced Camera for Surveys to study the Hα emission properties of main-sequence stars in the globular cluster 47 Tucanae. Using a combination of multiband observations in the F606W, F814W and F658N bands, we search for stars showing Hα excess emission. An accurate photometric measurement of their Hα equivalent width allows us to identify objects with large Hα emission, which we attribute to mass accretion rather than enhanced chromospheric activity. The spatial position of some of these stars is coincident with that of known X-ray sources, and their location in the colour–magnitude diagram allows us to classify them as active binaries or cataclysmic variables (CVs). We show that this method, commonly adopted to study accreting discs in young stellar objects, can be successfully used to identify and characterize candidate CVs.